1. Field of the Invention
The present invention generally pertains to the field of methods of making printed circuit boards. In particular, the present invention pertains to method of making printed circuit boards having high conductor track densities.
2. Discussion of the Related Art
Various methods or producing printed circuit boards have been presented.
For instance, DE 32 45 272 discloses a method for producing miniaturized thick- and thin-film circuits in which, for the purpose of significantly increasing the conductor track density first of all at least on those regions of the substrate at which a miniaturized conductor track geometry is provided, a whole-area layer of conductive material is applied and this layer is then subdivided by means of a laser. The laser is controlled according to a negative layout program and is aligned with respect to the substrate into mutually separate partial areas and/or strips which at least in part form a conductor track. When a multiplicity of IC pads are connected to a surrounding periphery, diverging tracks burnt by the laser result in the production of conductor tracks which diverge in a star-shaped manner. The tracks are very narrow on the IC side and are each assigned to one of the closely adjacent pads and the widened outer ends of which lead to periphery connections. As a result of the track burnt by the laser being routed in the shape of a spiral, inductances can also be formed in the conductive layer in a whole-area manner. In this case, a highly compact and thus space-saving inductance structure can be obtained by the spiral arrangement of the conductor track located in each case between two tracks burnt by the laser.
Also, EP 0 602 258 discloses a method for producing printed circuit boards having coarse conductor structures. A delimited region of these printed circuit boards is intended to obtain a very high wiring density. This is achieved by means of an additional wiring layer which is applied only in the delimited region and is connected to the underlying wiring layer via plated-through holes.
In EP 0 062 300, a method for producing printed circuit boards is disclosed in which a metallic etching resist that has been applied over the whole area of a metal layer is selectively removed by means of laser radiation in the regions which do not correspond to conductor structures. The conductor structures are formed by etching away the uncovered metal layer.
Furthermore, DE 41 31 065 discloses a method for producing printed circuit boards in which a metal layer and a metallic or organic etching resist layer are successively applied to a substrate. The etching resist layer is then removed by means of laser radiation in the regions directly adjoining the subsequent conductor track pattern, and the uncovered metal layer is etched away so that the conductor track pattern and island regions of the metal layer remain on the substrate. The island regions are electrically insulated from the conductor track pattern by etching trenches. Patterning by means of laser radiation can be performed rapidly since the regions of the etching resist layer which are to be removed only have to have a small width and the larger areas remain between two conductor tracks.
In principle, the production of the fine conductor structures would also be possible using conventional but costly photoetching technology. In this case however, high-quality photoresists, high-quality light sources and high-quality photomasks, would have to be used, and the work would have to be carried out in dust-free rooms. All these measures can be obviated by the application of laser patterning for the fine conductor structures, while at the same time giving a very high yield.
However, the present invention solves the problem of producing printed circuit boards having a high conductor track density in at least one delimited region, in an economical and simple manner.
The present invention combines conventional photoetching technology for producing the coarse conductor structures and laser patterning for producing the fine conductor structures. The method sequences can be coordinated with one another in such a way that the formation of the coarse and fine conductor structures can be performed in a common etching process.
For the purposes of discussing the present invention, coarse conductor structures are structures which have conductor track widths and conductor track spacings of more than 100 um, whereas fine conductor structures are structures which have conductor track widths and conductor track spacings of 100 um or less.
Accordingly, it is an object of the invention to provide a method of making printed circuit boards having plated-through holes both in a region of coarse conductor structures and in a region of fine conductor structures.
It is another object of the invention to provide a method of making printed circuit boards having extremely reliable plated-through holes through the metallic reinforcing layer in the region of the coarse conductor structures.
It is a further object of the invention to provide a method of making printed circuit boards in which protection is provided for the region having one conductor structures during the deposition of the metallic reinforcing layer in the region of the coarse conductor structures.
It is an additional object of the invention to provide a method of making printed circuit boards in which photoresist is patterned with the aid of a laser beam in the region of fine conductor structures.
It is yet another object of the invention to provide a method of making printed circuit boards in which an etching resist can be applied to the coarse and to the fine conductor structures in one work operation.
It is yet a further object of the invention to provide a method of making printed circuit boards in which the region having fine conductor structures is protected during the application of the metallic reinforcing layer by a temporary mask.
It is yet an additional object of the invention to provide a method of making printed circuit boards that easily enables direct laser patterning of the etching resist in the region of the fine conductor structures.
These and other objects of the present invention will become apparent upon careful review of the following disclosure, which is to be read in conjunction with review of the accompanying drawing figures.